Species vary phenotypically and genetically across their environmental range limits, and this variation can influence ecological processes. Ecologically meaningful intraspecific variation might be particularly important in the context of agricultural weeds and exotic invaders, because intraspecific variation in these species might allow them to rapidly adapt to their unusually dynamic and variable environments. In a greenhouse study, we explored intraspecific variation in the size, rhizome production, and competitive ability of the global invader, Johnsongrass (Sorghum halepense), representing populations from agricultural and non-agricultural habitats across its introduced North American range. We also used these populations to explore the relationship between phenotypic variation and genomic endoreduplication responses to the common stresses herbicides, competition, and clipping. Endoreduplication occurs when plants increase their genome size by increasing their nuclear chromosome number, with some evidence showing correlations with stress response. We found that Johnsongrass plants from agricultural habitats were larger than plants from non-agricultural habitats, but there was no difference between habitats in either rhizome production or competitive ability. Two of the five herbicides we tested, primisulfuron and imazethapyr, had the strongest suppressive effects on Johnsongrass, and also stimulated the greatest rates of endoreduplication. Furthermore, agricultural populations showed higher levels of endoreduplication. We found no overall effect of competition on endoreduplication, although endoreduplication was higher for non-agricultural populations than agricultural populations. When competing with corn, but not with conspecifics, Johnsongrass roots increased endoreduplication by 13%. Clipping induced substantial endoreduplication, but there was no difference between agricultural and non-agricultural populations. Our results suggest that endoreduplication may play a role in some, but not all, stress responses in Johnsongrass. Furthermore, our results indicate that Johnsongrass has adapted in some phenotypic and genomic ways to agricultural habitats in North America. Such adaptation may play a role in this species' success as both an agricultural weed and an exotic invader. / Master of Science in Life Sciences / Agricultural weeds and exotic invasive plant species cause substantial economic and ecological impacts. Exploring the ways in which these species thrive, spread, and cope with different forms of stress contributes to our understanding of why these plants can be so successful and cause such damage. We studied how one of the world’s worst weeds and invaders of natural systems, Johnsongrass (Sorghum halepense), responded via growth and by increases in the amount of DNA in its cells to different forms of stress. We also asked whether any Johnsongrass populations appeared to have adapted to agricultural habitats. We found that Johnsongrass plants grown from seed collected from agricultural fields produced larger plants than those from non-agricultural locations (e.g., roadsides), suggesting some degree of adaptation to agricultural environments by Johnsongrass. We also found that stress from sub-lethal doses of some herbicides suppressed the growth of Johnsongrass, simultaneously stimulating more cells with increased cell DNA content, a genomic process called endoreduplication. Interestingly, there were higher rates of endoreduplication in plants from agricultural populations in the herbicide experiment, suggesting that endoreduplication may help Johnsongrass cope with stress and that agricultural populations show adaptive increases in this genomic stress response. Competition from corn or other Johnsongrass plants suppressed target Johnsongrass plants, but did not stimulate more endoreduplication in the suppressed plants. Finally, clipping reduced the size of Johnsongrass substantially, and also stimulated increased rates of endoreduplication. In general, our results indicate that Johnsongrass has adapted in some ways to agricultural habitats in North America, and that this adaptation may play a role in this species’ success as both an agricultural weed and an exotic invader.

Invasive species are the second-leading cause of the decline of native species, making it a critical global environmental issue (Pimentel 2005). The goal of this study was to identify which non-native invasive plant species are common in eastern Travis County and which environmental factors associate with their establishment and spread in this area. Five parks and preserves were sampled, as they are intended for the conservation of native species and are more accessible for educational use. Six of the 15 species searched for, Bothriochloa ischaemum, Lactuca serriola, Ligustrum lucidum, Lonicera japonica, Melia azedarach, and Sorghum halepense, were each found in at least one of the five sites. The presences and absences of these species were compared to a suite of ecosystem properties, including habitat, disturbance, the average soil depth, and the horizontal and vertical distances to the nearest pond, lake, stream, or river. None of these six species appears to need observable recent disturbance. All of these six species, both woody and herbaceous, can tolerate shallow soils. The woody invasive species Ligustrum lucidum and Melia azedarach seem to prefer more mesic habitats. The invasive grass species Bothriochloa ischaemum and Sorghum halepense seem to prefer habitats that are more open. Several invasions of potential conservation concern were identified. / text